1. Field of the Invention
The present invention relates to a transmitting diversity communications apparatus.
2. Description of the Related Art
Transmitting diversity in W-CDMA, which is the third-generation mobile communications system, adopts a method using two transmitting antennas.
FIG. 1 shows an example configuration of a transmitting diversity system using two transmitting antennas.
Mutually orthogonal pilot patterns P1 and P2 are transmitted from two transmitting antennas 1 and 2, respectively, as pilot signals, and channel impulse response vectors h1 and h2 from each antenna of a base station up to the receiving antenna of a mobile station are estimated by correlating each known pilot pattern to an incoming pilot on the receiving side of the mobile station.
A control amount calculation unit 10 calculates and quantizes the amplitude/phase control vector (weight vector) w=[w1, w2] of each transmitting antenna of the base station that maximizes power P expressed by the following equation (1) using these channel estimation values. Then, a multiplex unit 11 multiplexes the quantized weight vectors with an uplink channel signal as feedback information and transmits the signal to the base station. However, since there is no need to transmit both values w1 and w2, it is acceptable to transmit only value w2 obtained by assigning w1=1.P=wHHHHw  (1)H=[h1, h2]  (2)
In equation (2), h1 and h2 are the channel impulse response vectors from the transmitting antennas 1 and 2, respectively, and the superscript H on HH and wH indicates the Hermitian conjugation of H and w, respectively. If an impulse response length is assumed to be L, the channel impulse response vector is expressed as follows.hi=[hi1, hi2, . . . , hiL]  (3)
Therefore, in the case of two transmitting antennas, equation (1) is calculated based on the following algebraic calculation.
  H  =                    [                                                            h                11                                                                    h                21                                                                                        h                12                                                                    h                22                                                                        ⋮                                      ⋮                                      ]            ⁢              ,                                      ⁢              w        _              =                                        [                                          w                1                            ,                                                          ⁢                              w                2                                      ]                    T                ⁢                  ,                                                ⁢                                  ⁢        therefore        ⁢                                  ⁢        Hw            =              [                                                                                                  h                    11                                    ⁢                                      w                    1                                                  +                                                      h                    21                                    ⁢                                      w                    2                                                                                                                                                                h                    12                                    ⁢                                      w                    1                                                  +                                                      h                    21                                    ⁢                                      w                    2                                                                                                          ⋮                                      ]            
At the time of handover, weight vector w that maximizes the following equation is calculated instead of equation (1).P=wH(HH1H1+HH2H2+ . . . )w  (4)
In equation (4), Hk is a channel impulse response signal from the k-th base station.
Then, the feedback information extraction unit 12 on the transmitting side extracts w2 (in this case, w1=1 is assumed) transmitted from a mobile station, from an incoming signal and an amplitude/phase control unit 13 multiplies a data signal to be transmitted from the transmitting antenna 2 by w2. In this way, the degradation of both the amplitude and phase of signals received from the transmitting antennas 1 and 2 that are received on the receiving side are corrected in advance and are transmitted from the transmitting side.
In W-CDMA, two methods are stipulated: mode 1 for quantizing weight coefficient w2 into one bit and mode 2 for quantizing w2 into four bits. In mode 1, since control is exercised by transmitting one bit of feedback information for each slot, control speed is high. However, since quantization is rough, accurate control is impossible. In mode 2, since control is exercised by transmitting four bits of information, more accurate control is possible. However, in mode 2, since only one bit can be transmitted for each slot and feedback information of one word is transmitted for every four slots, control cannot track fading in the case of a high fading frequency, and amplitude/phase characteristics degrade. As described above, if the signal transfer rate of an upward channel from a mobile station to a transmitting station, for transmitting feedback information is restricted, control accuracy and fading track speed have an inverse relationship.
The Release-99 specification of W-CDMA standard does not take into consideration a case where more than two transmitting antennas are used so as to avoid the degradation of uplink channel transmission efficiency due to feedback information transmission. However, if the increase of feedback information or the degradation of update speed is allowed, the number of antennas can also be increased to three or more. In particular, currently a case where four transmitting antennas are used is being extensively researched and developed.
If a closed-loop transmitting diversity system is applied to the radio base station of a cellular mobile communications system, a signal from each transmitting antenna independently suffers from fading, and ideally the same phase combination is performed at the antenna position of the mobile station. Therefore, a diversity gain corresponding to the number of transmitting antennas can be obtained and the gain can also be improved by the combination. Accordingly, the receiving characteristic is improved and the number of users accommodated in one cell can also be increased. “Ideally” means a case where there is neither transmission error of feedback information, control delay, channel response estimation error nor quantization error of a control amount. In reality, the characteristic degrades due to these factors compared with that of the ideal case.
In order to obtain a diversity gain corresponding to the number of antennas, antenna spacings (the distances between antennas) must be wide so that fading correlation may become sufficiently low. Generally, in order to suppress fading correlation to a sufficiently low level in the radio base station of a cellular mobile communications system, antenna spacings must be approximately 20 wavelengths. Since one wavelength is approximately 15 cm in a 2 GHz band, antennas must be installed approximately 3 meters apart. Therefore, if the number of transmitting antennas increases, an area needed to install antennas becomes wide and it becomes difficult to install antennas on the roof of a building and the like, which is a problem. Diversity gain is saturated as the number of transmitting antennas increases. Therefore, when the number of transmitting antennas reaches a specific value, the diversity gain cannot be improved any further even if the number of transmitting antennas is further increased.
When the number of transmitting antennas is increased, an amount of information to be fed back increases since feedback information must be transmitted to each antenna. Therefore, in that case, the transmission efficiency of an uplink channel degrades due to feedback information transmission or the control of transmitting diversity cannot track high-speed fading. As a result, the characteristic degrades, which is another problem.